The invention relates to rotary drill bits for use in drilling deep holes in subsurface formations.
In particular, the invention relates to drill bits of the kind comprising a bit body providing a leading face, a gauge region, and an annular space (known as the annulus) on the side of the gauge region remote from the leading face, a number of blades on the leading face, each blade having an outer surface which, in use, faces the surface of the formation being drilled and a front suface facing in the direction of normal forward rotation of the bit, a number of cutting elements mounted along each blade, a delivery passage in the bit body for supplying drilling fluid to a number of openings in the leading face of the bit, and at least one exit passage for returning said drilling fluid past the gauge region to the annulus. Said openings are so located on the forward side of a blade as to promote a flow of drilling fluid across the front surface of the blade and across the cutting elements mounted on the blade. The exit passage is usually in the form of an exit channel or junk slot formed in the outer surface of the gauge region, but the invention also includes arrangements where other forms of exit passage are employed.
A drill bit of this kind is described in U.S. Pat. No. 4,499,958. In the drill bit described in that specification, there are provided four blades and two openings for drilling fluid on the forward side of each blade, the openings being at different radial distances from the axis of rotation of the bit. There are provided four junk slots each of which extends, around the periphery of the bit, from the front surface of each blade to a location spaced a short distance from the rear of the next blade. Accordingly, almost the whole of the space between adjacent blades is in direct communication with the associated junk slot. Such an arrangement provides substantial clearance and a large flow path for the escape, through the junk slot, of cuttings produced by the cutting elements and it is believed that this is one of the reasons for the high penetration rates which drill bits of this kind may achieve in certain types of formation. However, a further important function of the drilling fluid is to cool the cutting elements and the formation on which the cutting elements are acting, and with the arrangement described in U.S. Pat. No. 4,499,958 such cooling may not be particularly effective in view of the fact that drilling fluid emerging from the nozzles in the bit body can flow directly to the junk slot without providing sufficient high velocity turbulent flow of fluid over the cutting elements and the formation to provide effective cooling.
The present invention provides an improvement to a drill bit of the kind first referred to, the improvement comprising the provision of a structure which controls the flow of drilling fluid emerging from the openings in the bit body in such manner as to promote a controlled vortex of flow on the forward side of each blade to give a greater velocity gradient to the flow over the cutting elements and thereby provide more effective cooling of the cutting elements and the formation on which they are acting.
It should be explained that in most conventional drill bits of the kind first referred to above, including the type described in U.S. Pat. No. 4,499,958, the flow of drilling fluid over the surface of the bit body and cutting elements is complex, and it may be assumed that such flow will also involve the generation of one or many small vortices, but in a generally uncontrolled manner. The present invention, however, depends on the production of a controlled vortex, that is to say the configuration of the bit body and the location of the openings therein for drilling fluid are such as inevitably to result in the generation of at least one major vortex in the region adjacent the front surface of the blade, the peripheral region of the vortex passing across several or all of the cutting elements mounted on the blade.